1. Field of the Invention
The present invention relates to a piezoelectric device, and more particularly, to a piezoelectric device in which an element portion, such as a resonator or a filter, is disposed on a piezoelectric substrate.
2. Description of the Related Art
A piezoelectric device has been proposed in which in which an IDT electrode disposed on a piezoelectric substrate is covered with a cover layer.
For example, in a piezoelectric device 110 shown in a cross-sectional view in FIG. 8, on one main surface 111a of a piezoelectric substrate 111 having a conductive pattern including an IDT electrode 112, and pad electrodes 113, a frame-like supporting layer 120 made of a resin is arranged so as to surround an IDT-forming region where an IDT electrode 112 is arranged, and a cover layer 130 made of a resin is disposed on the supporting layer 120. Furthermore, the piezoelectric device 110 is entirely covered with an outer resin 140, and an IDT space 114 surrounding the IDT electrode 112 is sealed (see, for example, Japanese Unexamined Patent Application Publication No. 2006-352430).
In the piezoelectric device in which the IDT electrode is covered with the cover layer as described above, as shown in a perspective view of FIG. 6, the supporting layer 120 which supports the cover layer is spaced apart from the IDT electrode 112 so as not to adversely affect the region in which vibration propagates on the piezoelectric substrate 111 (the region in which the IDT electrode 112 is formed and its vicinity). The coefficient of linear expansion of the supporting layer 120 made of a resin is greater than the coefficient of linear expansion of the piezoelectric substrate 111. Therefore, when there is a change in temperature, thermal stress occurs between the piezoelectric substrate 111 and the supporting layer 120, and under the influence thereof, strain occurs in the region in which vibration propagates on the piezoelectric substrate 111, thus changing the vibration propagation state. As a result, temperature characteristics of the piezoelectric element including the IDT electrode 112 are degraded.
The degradation in temperature characteristics is believed to be improved by increasing the distance between the IDT electrode 112 and the supporting layer 120.
However, when a structure in which a supporting layer 120 is disposed close to an IDT electrode 112 on a piezoelectric substrate 111, as shown in FIG. 7A which is a cross-sectional view of a main portion, is changed to a structure in which a supporting layer 120x is spaced away from the IDT electrode 112 such that the distance between the IDT electrode 112 and the supporting layer 120x is increased as shown in FIG. 7B which is a cross-sectional view of a main portion, a cover layer 130 on the supporting layer 120x sags and attaches to the IDT electrode 112, which degrades characteristics. In particular, in the case in which molding is performed with a resin, the cover layer 130 easily sags under pressure during molding, and characteristics are significantly degraded.
Furthermore, because of the increase in the distance between the IDT electrode and the supporting layer, design freedom is reduced. In addition, the size of piezoelectric devices cannot be reduced.